When designing microwave circuits, microstrip transmission lines are commonly used. A microstrip transmission line comprises a metal ground plane and a conductor, where a dielectric carrier material is positioned between the metal ground plane and the conductor. This configuration is economical and relatively easy to design.
However, due to losses in the dielectric carrier material, it is sometimes not possible to use microstrip transmission lines. When there for example is a filter in the layout, the filter may have to be realized in waveguide technology. Waveguides are normally filled with air or other low-loss materials. When there is a filter in a microwave circuit microstrip layout, the filter may thus be realized by means of a waveguide filter in order to lower the losses.
There is also a possibility that a signal that is carried by a microstrip transmission line is intended to be radiated by a radiating element, such as for example an antenna.
There are also a number of other circumstances where a microstrip transmission line is transformed to a waveguide interface, for example when a diplexer or a triplexer is used.
In these cases, the microstrip signal is often first fed through an integrated circuit, normally comprising a so-called chip, before being fed to a waveguide interface. For example, the chip is based on GaAs and comprises an LNA (Low Noise Amplifier), or some other circuitry having a desired electrical functionality. A typical such integrated circuit is mounted in a so-called QFN (Quad Flat No Lead) package which comprises a number of input leads, output leads, ground leads, control leads and bias leads.
There is thus a desire to obtain a transition from such a package to a waveguide interface. The most common way is to solder the package leads to a dielectric carrier material, for example in the form of a PCB (Printed Circuit Board) made from any suitable kind of laminate material, where the package has an output lead that is connected to a probe. The probe is arranged in such a way that it feeds a waveguide and/or an antenna, where the waveguide may be in the form of a surface-mounted waveguide.
Such a surface-mounted waveguide is normally made having three walls and one open side. Metalization is then provided on the side of the dielectric carrier material facing the waveguide, where the metalization serves as the remaining wall of the waveguide, thus closing the waveguide structure when the waveguide is fitted to the dielectric carrier material.
However, when the frequencies used lie around 60-90 GHz, the losses will be too high, resulting in that a satisfactory result is not possible to achieve. The mentioned QFN package, or other similar packages available today, does not present a satisfactory performance in this frequency interval. This is due to the fact that the connections from the chip itself to the package leads normally are wire-bonded with a bond wire. Such a wire-bonding constitutes a large inductance, and induces heavy losses at 60-90 GHz.
There is thus a desire for a transition from a microwave chip to a waveguide interface, where the losses are minimized.